1. Technical Field
This invention relates generally to spark plugs and other ignition devices, and more particularly to electrodes having platinum firing tips and to a method of construction thereof.
2. Related Art
Within the field of spark plugs, there exists a continuing need to improve the erosion resistance and reduce the breakdown voltage between the spark plug's center and ground electrodes. Various designs have been proposed using noble metal electrodes or, more commonly, noble metal firing tips applied to standard metal electrodes. Typically, the firing tip is formed as a pad or rivet which is then welded onto the end of the electrode.
In constructing firing tips with noble metals, there also exists a continuing need to improve the reliability of the attachment of the noble metal firing tip material to the electrode material, which is often constructed from a nickel alloy. For example, in U.S. Pat. No. 6,132,277, which is assigned to the assignee of the applicant herein, a precious metal is placed on a planar surface of the electrode, resistance welded, then resistance welded thereto. Further, the desired shape of the precious metal firing tip is preferably formed after resistance welding, and can then be resistance welded again to further secure the firing tip to the electrode which may have been loosened during the forming process or may not have been firmly attached during the initial resistance weld.
In U.S. Pat. No. 5,811,915, another construction of a spark plug having a precious metal chip secured to an electrode is disclosed. The '915 patent teaches attaching a noble metal chip formed of iridium, or an alloy thereof, by first resistance welding the chip to the electrode. During the resistance welding process, the noble metal chip remains unmelted, and is pushed toward the electrode so that it sinks into the melted electrode material, thereby forming protruding portions about an outer perimeter of the chip. Subsequently, a laser beam is applied to a point location, shown as being two points generally opposite one another, on the protruding portion of the electrode at an incident angle of 45 degrees to melt the impinged protruding portion of the electrode and a side surface of the noble metal chip in the vicinity of the protruding portion. Accordingly, a laser weld joint extends into a side surface of the precious metal chip above its lower surface which was previously sunk into the melted electrode material. Then, another peripheral laser weld is performed entirely along the outer periphery of the noble metal chip by rotating the electrode about its axis.
In U.S. Pat. No. 6,705,009, another construction of a spark plug having a precious metal secured to a center electrode is disclosed. The '009 patent teaches attaching a flat end of a continuous precious metal wire to a flat end of a tapered ignition tip of the center electrode via a first resistance or friction weld. During the first weld, the end of the wire forms a flat butt-weld joint with the end of the center electrode. The wire is then cut, and a second weld is formed via a laser about the outside periphery of the first weld joint between the cut wire and the center electrode.
In U.S. Pat. No. 6,819,031, another construction of a spark plug having a precious metal firing tip secured to an electrode is disclosed. The '031 patent teaches attaching a noble metal chip to a center electrode via a temporary resistance weld or a jig, and then forming a laser weld around a full circumference of the interface of the noble metal chip and the center electrode to form a first weld layer. Then, the laser is shifted along the longitudinal axis of the center electrode to form a second weld around the full circumference of the interface, with additional weld layers being possible thereafter, with each additional weld layer being shifted axially along the longitudinal axis of the electrode.
In U.S. Pat. No. 6,827,620, another construction of a spark plug having a precious metal secured to an electrode is disclosed. The '620 patent teaches attaching a noble metal chip to a center electrode via a provisional resistance weld, and thereafter forming a final laser weld. The noble metal chip is a pillar shaped element of iridium, or an iridium alloy material. During the provisional resistance welding, the chip is pressed with sufficient force to embed an unmelted portion of the pillar shaped chip into the electrode preferably not more than 0.1 mm.
Of all the known electrode constructions having a precious metal firing tip, including those discussed above, each comes with potential drawbacks. Some of the possible drawbacks include, increased costs in manufacture, a limited number of types of firing tip materials available for use, or a combination thereof. As such, the subject invention seeks to remedy these and any other potential problems present in the known constructions.